As is known, water has become an essential resource for our planet and may be a worldwide strategic asset. The increasing demand of water, especially for agricultural and industrial use has been addressed by novel technologies, which are aimed at reusing and recycling process and waste waters from civil and industrial purification plants.
EP 1 129 757, by the applicant hereof, discloses a machine for microfiltration of suspended solid particles from a liquid stream, having all the features of the preamble of claim 1.
Particularly, the machine as disclosed in the above mentioned patent comprises a box-like enclosure with an inlet conduit for receiving the liquid to be filtered, which defines a flow direction, in which such caisson has therein an influent chamber for containing the liquid to be treated, a treatment chamber located downstream from the influent chamber and having an at least partially cylindrical bottom wall, at least one pair of filter disks, which are keyed to a shaft substantially perpendicular to the flow direction, said disks having inner surfaces adapted to delimit the treatment chamber and peripheral edges in sliding contact with the bottom wall, at least one effluent chamber for receiving the filtered water, located downstream from the treatment chamber and delimited by the pair of filter disks, a discharge conduit for discharging the liquid contained in the effluent chamber and formed in the enclosure. There are provided drive means which act upon the shaft and/or the filter disk to impart a rotation to the pair of disks in a predetermined direction, jet washing means directed against the outer surfaces of the disk for removing the particles retained thereby, means for conveying the solid particles removed by the washing means, comprising at least one first longitudinal duct interposed between the disks and having substantially parallel longitudinal edges placed at a minimum distance from the inner surfaces of the disks, for collecting the solid particles so removed.
This known machine, which is manufactured and sold by the applicant under the trademark ULTRASCREEN®, uses disks with filter meshes operating with the principle of “dynamic tangential filtration”, which implies that, as the disks rotate, the direction of the incoming flow is substantially parallel to the filter disks, whereby the particles that move over them have a velocity with a main component parallel to the flow and a second component perpendicular thereto, i.e. with a resultant of the velocity vector inclined to the planes of the meshes.
As a result, the actual flow passage section of the mesh is smaller than the theoretical front section, due to the inclination of the relative velocity of the particles. Therefore, as the rotation speed of the disks increases, the flow passage section of the mesh decreases. This will provide the dynamic effect of blocking the passage of particles having a smaller size than the individual mesh apertures, thereby increasing the effectiveness of the filtration process.
Therefore, as larger particles are blocked, a film is formed, which acts as a precoat that increases filtration efficiency. While this prior art machine perfectly fulfills its intended purpose, it is still susceptible to further improvement. Solid particles deposited on disk meshes are periodically removed by jet washing and backwashing means, directed against the outer surfaces of the disks, to urge them toward a collecting duct interposed between the disks and convey them outside, using appropriate manifolds.
Since each disk has a considerable maximum diameter, typically above 500 mm, its surface, and particularly the surface of the filter mesh of the disks is rather irregular, and the edges of the ducts cannot be maintained at a distance of less than 20-30 mm to prevent interferences with the mesh or damages thereto. Therefore, part of the solid particles retained by the disks pass through the gap between the edges of the duct and the inner surface of the disks and fall into the treatment chamber, which will reduce filtration effectiveness and will not allow the amount of solid particles suspended in filtered water to fall within given values.
Furthermore, since national regulations about reuse of wastewater, such as the California Health and Safety Code of Regulations, Water Code, Titles 22 and 17, are stricter and stricter and require the total solid suspended particle content (SST) to be less than 5 mg/l, the problem arises that these filtration conditions must be improved for allowing recycling of wastewater for agricultural and industrial uses with the maximum possible effectiveness.
A further feature of this prior art machine is a particular massive and heavy structure of the disks, for withstanding considerable hydraulic loads and high flow rates, which increases the inertia of the rotor and hence the power of the run motor, the energy consumption at startup and in steady-state conditions, and transport and maintenance requirements.
Further improvable aspects are the effective efficiency of the filter disk backwashing means, and the lack of protection against splashing of backwashing fluid jets.